Vertically-oriented optical devices may comprise multi-layered reflecting structures such as distributed Bragg reflector (DBR) mirrors forming a resonant cavity perpendicular to a surface of a semiconductor wafer on which the devices are fabricated; and these optical devices may include both light-emitting devices such as vertical-cavity surface-emitting lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs), and also light-detecting devices such as resonant-cavity photodetectors (RCPDs). Such vertically-oriented optical devices are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications.
Vertically-oriented light-emitting and light-detecting optical devices have very similar device structures comprising an active region sandwiched between a pair of DBR mirrors with a semiconductor p-n or p-i-n junction formed about the active region. When an electrical bias voltage is applied across the junction, light may be generated within the active region for a forward biasing of the junction; and light received into the active region may be detected for a reverse biasing of the junction.
A particular problem in the development of vertically-oriented light-emitting devices, and especially for VCSELs, has been a substantial voltage drop across the DBR mirrors as compared to the voltage drop across the active region wherein light is generated. Each DBR mirror is a unipolar heterostructure consisting of alternating one-quarter-wavelength thick layers of two different semiconductors of the same conductivity type (e.g. n-type or p-type) having different indices of refraction. Heterojunction energy band discontinuities between adjacent semiconductor layers in each DBR mirror may form potential barriers between the semiconductor layers due to offsets in a conduction band, or a valence band or both that gives rise to the voltage drop across the DBR mirror. For a VCSEL, this voltage drop increases a bias voltage across the device and produces excess heating therein, thereby deteriorating device efficiency and performance. Attempts have been made to reduce the voltage drop due to the DBR mirrors (especially the p-type mirror) in VCSELs by composition grading a region about each heterojunction in the mirrors so that a semiconductor alloy composition is smoothly graded from each semiconductor layer to an adjacent semiconductor layer.
The teaching of the prior art as disclosed, for example, in U.S. Pat. No. 5,170,407 to Shubert et al and in U.S. Pat. No. 5,379,719 to Chalmers et al has been to vary the composition on both sides of each heterojunction in the DBR mirrors in a light-emitting device in a gradual (i.e. continuous) manner (as averaged over an electrostatic screening distance or Debye length of carriers such as electrons or holes), and to avoid any discontinuities or abrupt changes in composition about the heterojunction. The teaching of the prior art has further been to eliminate any possible asymmetries within the DBR mirrors, including asymmetries about the heterojunctions therein. To this end, it has been taught in the prior art that a composition-graded region on each side of a heterojunction should be treated substantially identically, including making each region substantially identical in thickness in the growth direction and uniformly composition grading each region symmetrically about a point of inversion located at a midpoint of the heterojunction between the semiconductor layers of a DBR mirror. Furthermore, the prior art teaches that composition grading should take substantially the same shape on each side of the heterojunction (e.g. linear, piecewise linear, or semiparabolic) to smooth out or flatten the energy band on each side of the heterojunction.
Continuously-graded heterojunctions as required in the prior art are difficult to grow, especially for transitions from a binary semiconductor alloy composition (e.g. GaAs) to a ternary semiconductor alloy composition (e.g. Al.sub.x Ga.sub.1-x As) as may be required for continuously grading a heterojunction that includes at least one binary alloy composition therein. This difficulty arises from the need to grow a semiconductor alloy having less than about 10% of a constituent element, for which uncertainties in composition and growth rate may occur due to limitations of present day epitaxial growth methods such as molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). For example, in the case of MBE, such uncertainties may arise from a low temperature of a source cell needed to provide a low beam flux of a minority constituent element of the ternary alloy, and a nonlinear dependence of the beam flux on the source cell temperature. Additional uncertainties in composition and growth rate may occur from repeated ramping (i.e. a cyclical variation) of the source cell temperature for growing of a plurality of semiconductor layers and graded heterojunctions to form a DBR mirror.
Another method for grading heterointerfaces within DBR mirrors by MBE has been the use of rapid shutter sequencing to alternately grow many alternating very thin layers of a first binary alloy composition (e.g. GaAs) and a second binary alloy composition (e.g. AlAs) forming a superlattice having nanometer-scale layer thicknesses, with the thickness of each binary alloy layer being varied uniformly for generating an approximation to a smooth continuous composition grading about each heterojunction in the mirrors. (Such a binary alloy superlattice as seen by electrons and holes may approximate a smooth continuous composition graded heterojunction if the thickness of each binary alloy layer is smaller than a tunneling distance for the electrons or holes.) However, such rapid shutter sequencing is disadvantageous, especially from a manufacturing standpoint, due to the very large number (up to a thousand or more) of shutter operations that may be required for the fabrication of a light-emitting device having up to twenty or more heterojunctions within each DBR mirror. The prior-art MBE composition-grading approaches have been successful at reducing the voltage drop and series electrical resistance in DBR mirrors, but often at the expense of other important properties of the mirrors.
When using MOCVD for growing DBR mirrors, composition and growth rate uncertainties may also arise due to the need to control a very small gas flow for providing a minority constituent element of a tarnary alloy (e.g. less than about 10%). Mass flow controllers used by MOCVD to regulate the flow of gases for growing the semiconductor layers have a limited dynamic range which makes it difficult to control a small flow of gas for smoothly grading a heterojunction between two binary-alloy semiconductor layers.
Due to the limitations of the prior art, DBR mirrors have been largely comprised of either all-binary alloy composition semiconductor layers or all-ternary alloy composition semiconductor layers. The all-binary alloy DBR mirrors were grown by MBE with rapid shutter sequencing; but this approach is undesirable for manufacturing. The all-ternary alloy DBR mirrors (e.g. alternating Al.sub.0.1 Ga.sub.0.9 As and Al.sub.0.9 Ga.sub.0.1 As semiconductor layers with smoothly graded ternary alloy heterojunctions therebetween) were grown by MOCVD with mass flow control, or by MBE with source cell temperature ramping; and these approaches are undesirable due to the reduced reflectivity of each pair of semiconductor layers, and the resultant need for additional pairs of semiconductor layers to form a high reflectivity DBR mirror. In addition, the all-ternary alloy DBR mirrors have poorer thermal properties than the all-binary mirrors due to an increased alloy scattering. The higher thermal resistivity of ternary alloy semiconductors as compared to binary alloy semiconductors is disclosed, for example, in an article by S. Adachi entitled "GaAs, AlAs, Al.sub.x Ga.sub.1-x As: Material Parameters for Use in Research and Device Applications", published in Journal of Applied Physics, volume 58, pages R1-R29, Aug. 1, 1985 (see especially FIG. 12 therein).
What is needed is a method for fabricating optical devices comprising DBR mirrors that optimizes these mirrors not only to provide a low voltage drop therein, but also to optimize the mirrors for many other factors that affect device efficiency and performance, including lateral electrical resistance, thermal resistance, layer uniformity, and manufacturability. Such a comprehensive approach for fabricating DBR mirrors has been lacking in the prior art.
An advantage of the optical device and method of the present invention is that an optical device having one or more DBR mirrors may be fabricated considering optical, electrical, and thermal properties of the mirrors, with these properties substantially optimized for overall device efficiency and performance.
Another advantage of the optical device and method of the present invention is that one or more abrupt changes in composition about a heterojunction may be used to advantage to reduce uncertainties in growth rate, and to reduce a lateral electrical resistance to current flow within the device, thereby improving device efficiency and performance.
A further advantage of the optical device and method of the present invention is that an MOCVD growth method suited to volume manufacturing may be used to fabricate optical devices having a high efficiency for light generation.
These and other advantages of the optical device and method of the present invention will become evident to those skilled in the art.